A commenter whose brother-in-law worked as a restaurant manager
gave an estimate:

"I called him, he said that it differs restaurant-to-restaurant.
His store buys the shrimp wholesale at $4.55/lb for 31-35 shrimp
per pound. He said that the other ingredients, including fry oil
are negligible, and probably a few pennies per pound of shrimp.
He guesses total cost to you on your plate is around $5/pound,
including plates, cook's pay, etc.

So, at $5/pound, you'd need to eat a little over 3 pounds, or
about 100 shrimp, to make it a net loss. He said even if you came
in and ate 100 shrimp they would probably still make money
because you will come with other people or you will order a drink
or something. He has had two customers that have done it."

In other words, it's pretty much impossible for Red Lobster to
lose money on your "Endless Shrimp" order.